A typical planar waveguide includes a core through which light propagates, a first cladding that reflects the light in a state of being joined to an upper surface of the core, and a second cladding that reflects the light in a state of being joined to a lower surface of the core.
In following Patent Literature 1, a planar waveguide which is configured, by using, as each of first and second claddings, a material whose index of refraction is lower than that of a core, in such a way that light incident upon the core is totally reflected at the interface between each of the first and second claddings and the core is disclosed.